For driving an electrical load, such as an electrical motor, from an electrical grid, an electrical converter may convert a possible multiphase current from the grid into a possible multiphase current to be supplied to the load.
Such electrical converters are also used for supplying the electrical energy generated by a power source into an electrical grid. For example, a current of variable frequency, which is generated by a generator of a hydroelectric power plant or of a wind turbine, may be converted into a current of fixed frequency to be supplied to the grid.
Furthermore it is possible to interconnect two electrical grids with an electrical converter. In this case, the power flow may be in both directions.
Modular multilevel converters are a special type of electrical converter that may be especially suited for converting current of high energy and high voltage. Such a converter has a modular design and comprises converter cells connected in series. Each converter cell usually has two or four semiconductor switches and a cell capacitor. The power stored in the converter is not stored in a rather small number of DC link capacitors but in the cell capacitors, which are also subjected only to a smaller voltage compared to other types of converters.
For converting even higher currents with high voltages, it is possible to connect modular multilevel converters in parallel. Paralleling on converter level may be beneficial due to no or nearly no voltage/current derating compared to device paralleling and cell level paralleling, due to the fact that centralized inductor losses may be lower than distributed inductor losses compared to cell- and branch level paralleling and that more sinusoidal inductor voltages/currents usually result in fewer losses compared to branch level paralleling.